The image quality of an X-ray image produced by an X-ray device depends primarily on the dose of radiation that is incident on the X-ray detector when the X-ray is taken. The image quality is particularly good if the dose of radiation falls within a desired range predetermined by the design of the X-ray detector. The dose of radiation again depends firstly on a number of parameters of the X-ray device that can be set, in particular on the voltage and intensity of the X-ray current, the exposure time, etc. Secondly, the dose of radiation depends on the X-ray attenuation, that is, on the absorption of the X-rays by a subject that is arranged between the X-ray beam emitter and the X-ray detector of the X-ray device. In medical applications, the subject is generally a part of a patient's body that is to be examined. In such a case, the X-ray attenuation depends in particular on the size of the patient.